In recent years, wide use has been made of microprocessor based process control units for supervising industrial operations. Examples of such operations are the monitoring of the depth of material in silos and bins, the depth of fluid in channels, and the rate of flow of materials on conveyor belts. Typically, such equipment incorporates inputs from one or more transducers, connections to alarm or recording devices, and an operator interface consisting of some form of visual read out and some form of data entry keyboard which can be utilized for entering data into the unit. Electronically, the unit comprises a microprocessor associated with read only and random access memory and suitable peripheral interfaces so as to form a dedicated computer.
Such a unit most commonly operates under very adverse conditions in terms of the presence of dust, dirt, moisture and rough handling, and at the same time is also usually required to maintain calibration within fairly narrow limits. In this connection, the provisions for data entry in conventional units are a weak point, both since keyboards are vulnerable to the adverse conditions mentioned above, and because they may provide a means for unauthorized or inadvertent reprogramming or recalibration of the unit. Clearly there are measures which can be taken to alleviate these problems, but they increase the cost and complexity of the unit and its programming.
An example of a process control unit of the general class discussed above is described in U.S. Pat. No. 4,596,144 (Stanley Panton and Steven J. Woodward) entitled "Acoustic Ranging System", the specification of which is incorporated herein by reference. Reference to FIG. 1 of the drawings of that patent will show that it incorporates a computer 28 provided with various connections to transducers, indicators and displays, and receiving inputs from a keyboard 52 and control keys 58.
It has been proposed to connect the keyboard of a personal computer to the main unit of the computer by an infrared link. Such an arrangement was for example used in the IBM PC JR. (Trade Mark) computer. Numerous applications are also known in which portable keypads are utilized to provide remote control of domestic equipment such as television sets again using an infrared or ultrasonic link, although in the latter case, provision is invariably made for additional local control of the appliance.